The present invention relates generally to the time dependent measurement of optical pulses and, more specifically, to the time dependent measurement of terahertz (THz) frequency pulses using an optical streak camera.
Freely propagating terahertz (THz) pulses are usually measured by sampling techniques such as photoconductivc antenna or electro-optical (EO) sampling. Although these sampling techniques provide good signal-to-noise ratios and adequate temporal resolution, they cannot be used for measurement on a single-shot basis. Recently, a chirped pulse measurement technique, which is based on an electro-optic effect and wavelength division by multiplexing and demultiplexing, has been demonstrated for single-shot THz waveform measurement and one-dimensional imaging. Such progress makes it possible to study the unrepeatable events on a single-shot basis. Another electro-optical sensing technique is described in U.S. Pat. No. 5,952,818 issued to Zhang et al., incorporated herein by reference.
Theoretical analysis of the chirped pulse techniques shows, however, that temporal resolution of the chirped pulse measurement is given by the equation xcex94T=(Tc To)1/2, where Tc and To are the duration of the chirped and unchirped pulse, respectively. Therefore xcex94Txcx9c5 ps when Tc=100 ps and Toxcx9c0.25 ps. One of the factors limiting the temporal resolution in the chirped pulse measurement is the spectral bandwidth of the laser pulse. The THz pulse modulates the chirped probe pulse in the time domain, and the signal is extracted in the frequency domain. Because the chirped pulse technique is a frequency domain technique, the temporal resolution is limited due to the well-known time-frequency relation, and thus the technique is inherently limited in time resolution.
Further, because the frequency of the THz radiation limits time resolution, measurement of THz pulses faster than the theoretical time resolution requires measurement of the pulses in a time-dependent manner. Streak cameras provide optical measurements as a function of time. A streak camera measures ultrafast light pulses in the time domain using an electron tube. For direct measurement, photon energy should be high enough to free electrons from the electron tube cathode. Conventional streak cameras are only suitable for short wavelength measurement, such as X-ray, ultraviolet (UV), visible, and near infrared (IR) light, because of the material limitation of the cathode. With conventional streak cameras, longer wavelength radiation has not provided sufficient photon energy to free electrons from the cathode.
Many efforts have been made to extend the measurable radiation of conventional streak cameras to longer wavelengths. A far infrared streak camera was reported by using highly lying Rydberg state atoms. The measurable wavelength extended from near infrared to 100 xcexcm. See Drabbels et al., Opt. Lett. 22, 1436 (1997); Drabbels et al., IEEE J. of Quantum Electron. 34, 2138 (1998); and Drabbels et al., Appl. Phys. Lett. 74 (1999). In these experiments, UV laser source was needed to pump the electrons into excited slates, and the gas atoms must be in a vacuum chamber.
About ten years ago, the streak camera was used to measure radio frequencies and microwaves indirectly with an EO modulator as a converter. The radio frequency or microwave signals were converted into intensity modulation of a continuous-wave He-Ne laser. The highest measurable frequency was limited to about 40 GHz by the bandwith of EO modulator. See Chang et al., xe2x80x9cAn Electro-Optical Technique for Measuring High Frequency Free Space Electric Field,xe2x80x9d Fast Electrical and Optical Measurement, Edited by J. E. Thompson and L. H. Luessen, NATO ASI Series, Series E: Applied Sciencexe2x80x94No. 108, 57 (1983); and Williamson et al., xe2x80x9cPicosecond Electro-Electron Optic Oscilloscope,xe2x80x9d Picosecond Electronics and Optoelectronics, Ed. G. A. Mourou, D. M. Bloom, and C. -H. Lee, Springer-Verlag, 58 (1985). With the development of free-space EO sampling, the bandwidth has been extended to over 40 THz because the EO modulator is no longer a limiting factor. See Wu et al., Appl. Phys. Lett. 71, 1285 (1997); Han et al., Appl. Phys. Lett. 73, 3049 (1998); and Leitenstorfer et al., Appl. Phys. Lett. 74, 1516 (1999). In addition, the temporal resolution of a state-of-the-art streak camera is better than 200 fs.
Despite such improvements in the state of the art, there remains a need in the art to measure radiation pulses extending into the terahertz regime in a time-dependent manner.
To meet this and other needs, and in view of its purposes, the present invention provides a system for measuring a terahertz frequency pulse propagating in a free-space optical path. The system comprises an optical streak camera and an electro-optical modulator. The electro-optical modulator is positioned before the optical streak camera in the free-space optical path, and the streak camera measures optical intensity as a function of time. The electro-optical modulator includes an electro-optical crystal and a polarization analyzer.
This system can measure the terahertz frequency pulse in a single shot. The terahertz frequency pulse contains subpicosecond free-space electromagnetic radiation with a bandwidth in a range from 10 gigahertz to 40 terahertz.
Another embodiment of the present invention is a system for measuring terahertz frequency pulses with a streak camera that comprises an optical streak camera and an optical source with related optics for providing a pump pulse to excite an emitter to emit terahertz frequency pulses and a probe pulse. This system also has a probe pulse stretcher and a probe pulse polarizer, as well as a trigger that synchronizes the streak camera and the probe pulse. In addition, the system has an electro-optical modulator positioned before the optical streak camera in the optical path. This electro-optical modulator contains an electro-optical crystal and a polarization analyzer. The terahertz frequency pulse generates an electric field when the pulse propagates through the electro-optical crystal. This electric field modulates the polarization of the polarized probe pulse in the electro-optical modulator, which generates a polarization modulation. The polarization modulation is converted to an intensity modulation by the polarization analyzer, and the streak camera measures the intensity modulation.
The present invention also provides a method for measuring terahertz frequency electromagnetic pulses as a function of time. The method includes the steps of: providing optical pump and probe pulses; exciting a terahertz frequency emitter with the pump pulse; stretching the probe pulse; polarizing the probe pulse; modulating the probe pulse with a local electromagnetic field generated by the emitted pulse in an electro-optical modulator, which results in a probe pulse polarization modulation; converting the polarization modulation into an intensity modulation; and measuring the intensity modulation as a function of time. The method also includes synchronizing the optical probe pulse with the streak camera.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.